.gov is restricted to US government, but consider the immense sprawl that is the US federal government, and then consider that US state and US local governments are allowed in the club too, and it doesn't look so exclusive any more
.mil is restricted to the US military, but that is a gargantuan entity with countless agencies and bases and divisions and whatnot all of whom seem to want to have publicly accessible websites (even for stuff that is obviously only useful for people actually in the US military), and .mil turns out to be a dime a dozen too
.int – under current rules, you need to be an international organization established by international treaty, or else you need observer status with the UN General Assembly. Numerically that is smaller than either of the above two categories. (It also has some random stuff that doesn't belong under current rules, like the YMCA – which wasn't established by treaty, and doesn't have UN General Assembly observer status – but those are grandfathered registrations.)
“AQ domain names are available to government organisations who are signatories to the Antarctic Treaty and to other registrants who have a
physical presence in Antarctica.“
.arpa is in use, but only as an internal detail for reverse name resolution (i.e. looking up the PTR record for 1.2.3.4 queries the server 4.3.2.1.in-addr.arpa)
Which also yields one of the more interesting backronyms of the internet, as .arpa today is understood to mean "Address and Routing Parameter Area" as opposed to the Advanced Research Projects Agency that was involved in the early internet (ARPANET).
I believe anyone can register the .xyz domains. They're typically on sale for $0.99 on GoDaddy for the first year, so there seems to be a lot of junk, volume registrations using it.
My bad, my main point is that likely the most restrictive would be one of the corporate/private TLDs that are for internal use mainly. .xyz was a bad example. Maybe better is .bananarepublic
There's literally hundreds of new gTLDs like this that only have the one required nic.tld on them and nothing else (because they haven't launched yet and might never). My team runs a couple dozen of these.
For comparison's sake, we should probably restrict ourselves to legacy gTLDs, ccTLDs, and open, launched new gTLDs.
.gb, the old UK ccTLD, which still exists, but isn't open for registrations. AFAIK there are no websites in it, but you can ping hermes.dra.hmg.gb and friends.
I tried to search goo.gle on their whois and it seems like they even support private registration! All the fields are filled with "REDACTED FOR PRIVACY". Unaccessible except by Google, yet private!
.gov is restricted to US government, but consider the immense sprawl that is the US federal government, and then consider that US state and US local governments are allowed in the club too, and it doesn't look so exclusive any more
.mil is restricted to the US military, but that is a gargantuan entity with countless agencies and bases and divisions and whatnot all of whom seem to want to have publicly accessible websites (even for stuff that is obviously only useful for people actually in the US military), and .mil turns out to be a dime a dozen too
.int – under current rules, you need to be an international organization established by international treaty, or else you need observer status with the UN General Assembly. Numerically that is smaller than either of the above two categories. (It also has some random stuff that doesn't belong under current rules, like the YMCA – which wasn't established by treaty, and doesn't have UN General Assembly observer status – but those are grandfathered registrations.)